As weapons systems have developed, there have been substantial increases in both range and accuracy. However, systems that provide the greatest accuracy are extremely expensive, cumbersome to deploy and subject to failure because of their complexity. A need thus arises for a comparatively inexpensive weapons system that is reliable and accurate, yet lightweight and easy to use and deploy.
The trajectory of motion and hence the impact point of a nonguided projectile fired from an artillery piece such as a 155 mm Howitzer is determined by a relatively small number of parameters that are fairly well understood. These include the projectile shape, gun barrel characteristics, initial velocity of the projectile and atmospheric conditions. Typically all of these parameters will vary slowly over time, but if they can be determined accurately from one firing they will remain reasonably stable for a second firing shortly thereafter.
For example, the gun barrel typically contains helical grooves or rifling that causes the projectile to spin and attain greater stability and predictability as it exits that gun barrel. As the grooves wear, the impact on the projectile will change and the flight characteristics will gradually change with time.
Similarly, the exact initial muzzle velocity will change gradually with barrel characteristics, with temperature and with the powder charge that is used. The powder charge will be quite consistent within a manufacturing batch so that the initial muzzle velocities for two consecutive firings will be nearly the same. Atmospheric conditions including air density and wind velocity at different altitudes will tend to vary more rapidly than some of the other parameters. However, even atmospheric conditions will usually remain reasonably stable for many minutes at a time.
Radar systems have been developed to improve the accuracy of projectile firings. These systems tend to follow one of two configurations. One is a muzzle velocity radar system (MVR) that accurately determines the initial muzzle velocity. The other is a trajectory measurement radar system that tracks the trajectory of the projectile from firing to impact.
The muzzle velocity radar system is compact, lightweight and relatively inexpensive. However, it only provides accurate information as to one of the many parameters that determine the final impact point, namely initial muzzle velocity. This information significantly improves the accuracy of a firing, but leaves many important parameters to be approximated by other means. Examples of this type of system can be found in U.S. Pat. No. 4,837,718 to Alon and U.S. Pat. 3,918,061 to Elgaard.
The trajectory measurement radar systems use tracking radar systems with multiple sensor points located some distance from a weapon. These systems are large, complex and difficult to properly deploy, especially under battle conditions where time may be critical. Because of the complexity of these systems and the distances over which they must be deployed, their reliability is questionable. The high power radar signals that track the small projectiles from a substantial distance are subject to detection and tracking by enemy forces. However, because these systems track the complete trajectory of a projectile, they can be used to compile extremely accurate estimations of all of the parameters that affect the accuracy and final impact point of a projectile fired from a gun.
U.S. Pat. No. 4,679,748 to Blomqvist et al. discloses a system that monitors the trajectory of a guidable projectile. The projectile has flight control surfaces that are controlled in response to actual tracking information to guide the projectile to a desired impact point. The antenna is located off axis from the trajectory to provide the required trajectory information.